Unmanned aerial vehicles (“UAVs”) are remotely piloted or self-piloted aircraft that can carry cameras, sensors, communications equipment, or other payloads. A UAV is capable of controlled, sustained, level flight and is often powered by either a gas turbine or a reciprocating internal combustion engine. The UAVs may be remotely controlled or may fly autonomously based on pre-programmed flight plans or more complex dynamic automation systems.
UAVs have become increasingly used for various applications where the use of manned flight vehicles is not appropriate or is not feasible. Such applications may include military situations, such as surveillance, reconnaissance, target acquisition, data acquisition, communications relay, decoy, harassment, or supply flights. These vehicles are also used in a growing number of civilian applications, such as firefighting when a human observer would be at risk, police observation of civil disturbances or crime scenes, reconnaissance support in natural disasters, and scientific research, such as collecting data from within a hurricane.
Ducted fan vertical take-off and landing (VTOL) UAVs offer a distinct operational functionality in comparison to conventional fixed wing UAVs. This increased functionality is related to the ability of the Ducted fan VTOL UAV to hover and stare. In a hover and stare maneuver the UAV can be stopped in flight and any sensors on the UAV can be used to closely investigate a point of interest while the vehicle remains stationary.
A typical mission profile begins with the UAV ascending to a specified altitude. Once the UAV reaches its specified altitude, the UAV then cruises to a specified location and hovers at that location. Cruise, hover, and altitude changes may occur multiple times during a mission. The mission profile is completed with the UAV cruising to the landing location, descending, and landing at that location. Different power levels are required during the different portions of the mission profile. Currently, a gas turbine engine or a reciprocating internal combustion engine (“ICE”) are used to drive the rotating fan of the ducted fan propelled UAV. A gas turbine and an ICE are designed to produce peak efficiency at a specific power and speed, often referred to as the design point. The efficiency is reduced when the power and speed are varied from the design point. Throughout the mission profile, the engine is operated at many different power and speed conditions, resulting in less than optimum efficiency for certain legs of the profile. When the engine is not operating at optimum efficiency, higher fuel consumption results. Higher fuel consumption means the UAV cannot fly as far or as long as it could if the engine were operated at the design point throughout the entire mission profile.
Due to weight limitations, ducted fan UAVs typically have only one source of propulsive power. This is because two of any of the aforementioned power sources on a UAV would be too heavy of a load, resulting in decreased vehicle performance. However, if the one source of propulsive power fails to operate during a mission, or operates at a lower, uncontrolled manner, the result could be an uncontrolled flight or even a crash.
Also due to weight constraints, ducted fan UAVs with ICEs typically do not have an electrical starter or generator. Instead, electric power for flight is derived from an on-board battery. The battery level is slowly depleted during the mission. The depletion may limit flight time, thus limiting the utility of the vehicle. An ICE needs a significant torque applied to the crank shaft to be able to start. Typical small motors can supply high speed, but low torque. Without an electrical starter, the ducted fan UAV cannot land in a remote location with its engine turned off and then start up again to take off and resume the mission or return to base. This capability, commonly referred to as “perch and stare,” is desirable because it allows the vehicle to fly to a remote location and land while remaining able to transmit data, such as video and still images, back to the operator.